The invention relates to a circuit arrangement for an MR apparatus, having a resonant circuit, which is formed by an MR receiving coil and a capacitor, and having an electronic control circuit for switching the resonant circuit between two or more operating modes.
The invention also relates to an MR apparatus having a circuit arrangement according to the invention and to an MR method employing such an MR apparatus.
MR apparatuses used primarily for medical imaging purposes are known from the prior art; separate transmitting and receiving coil arrangements are provided in these apparatuses. The purpose of the transmitting coil arrangement here is to generate high-frequency fields in the examination volume of the MR apparatus, which induce nuclear magnetization in the examination volume. The receiving coil arrangements serve to pick up MR signals from the examination volume. The receiving coil arrangements can either be in the form of volume coils, or—for optimization of sensitivity in specific regions of the examination volume—in the form of surface coils, which are placed directly on the body of a patient located in the examination volume of the MR apparatus. Such receiving coils are normally wired up with suitably adapted capacitors, so that each receiving coil has associated with it a resonant circuit of which the resonant frequency is tuned to the proton resonant frequency of the MR apparatus. To induce the nuclear magnetization in the examination volume, high-frequency pulses in the power range of several kilowatts are normally introduced by means of the transmitting coil arrangement. The frequency of the high-frequency pulses is in turn the same as the resonant frequency of the resonant circuits associated with the receiving coils. The high-frequency pulses therefore enter resonantly into the receive-side resonant circuits of the MR apparatus. The result of this is that, on the one hand, the high-frequency energy is undesirably partially absorbed by way of the receiving coil arrangement. On the other hand, the resonant input possibly causes significant heating of or even irreversible damage to the receiving coil arrangement and to the corresponding resonant circuits. When using surface coils, the resonant heating may lead to a risk of injury for the patient being examined, on whose body the surface coils are placed directly. It is also problematic that the receive-side resonant circuits are customarily connected to sensitive receiver amplifiers, the input stages of which can be damaged by the high-frequency energy introduced in transmit mode. At any rate, the receiver amplifiers are severely overdriven, so that after every excitation by means of a high-frequency pulse the entire receive electronics of the MR apparatus have an undesirably long dead time before the received MR signals can be correctly processed.
In MR apparatuses that have separate transmitting and receiving coil arrangements, it is known specifically to detune the receive-side resonant circuits during transmit mode, so that an input of high-frequency pulses into the receiving coils during transmit mode is prevented. For that purpose, suitable circuit arrangements are provided, by means of which each of the receive-side resonant circuits can be switched back and forth between a resonant operating mode and a non-resonant operating mode. The resonant operating mode is here intended for the receive mode and the non-resonant operating mode is intended for the transmit mode of the MR apparatus.
A circuit arrangement of the kind outlined above is known, for example, from US 2004/0124838. The known circuit arrangement serves for switching a resonant circuit having a MR receiving coil in the form of a microcoil between a resonant and a non-resonant operating mode. Here, the entire resonant circuit is mounted at the tip of an intervention instrument. The known circuit arrangement comprises an optoelectronic element, by means of which the resonant circuit is detuned. In the case of the known circuit arrangement, the control signal that causes detuning of the resonant circuit is a light signal, which is fed to the optoelectronic element by way of an optical waveguide.
The drawback of the system known from the prior is primarily the fact that the (optical) control signal has to be fed to the resonant circuit to be switched back and forth between the different operating modes by way of a signal line specifically intended for this. For that purpose, apart from the optical waveguide, yet further optoelectronic components are required, which do not belong to the regular equipment of customary MR apparatuses. As a technically simpler version, it would admittedly be possible for the control signal for switching between the different operating modes to be fed to the resonant circuit in the form of an electrical signal by way of conventional electrical connecting wires. But the problem inherent in this is that suitable electrical connecting wires that connect the reversible resonant circuit of the MR receiving coil to the central control unit of the MR apparatus have to be several meters long. Such long wires in turn act as receiving antennas, which the high-frequency pulses couple into in transmit mode. Here too, resonant heating phenomena can occur, which represent a corresponding risk of injury for the patient to be examined.